I just finished reading an article in Guns and Ammo by Craig Boddington on the 30-06, good article by the way, and if I understood it correctly he states that indeed the 06 has a shorter neck, but this was done by moving the shoulder out. This allowed a 03 to be fired in a 06 chamber, but not the other way around. I wonder if some of the older 1895 -03s had a -06 reamer ran thru them and not marked? Don't have a dog in this fight, just reporting what I THINK I read. I've been wrong so many times I've quit counting.